Handles empty API_GATEWAY_BASE_PATH

[joelvh]

When API_GATEWAY_BASE_PATH is empty, this causes duplicate slashes at the beginning of the path (e.g. //my/path).

This update handles empty and null values for API_GATEWAY_BASE_PATH.

[codecov]

Codecov Report

Merging #14 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #14      +/-   ##
==========================================
- Coverage   99.81%   99.81%   -0.01%     
==========================================
  Files           5        5              
  Lines         536      535       -1     
  Branches       73       73              
==========================================
- Hits          535      534       -1     
  Misses          1        1              

Impacted Files	Coverage Δ
lib/serverless_rack.rb	100.00% <100.00%> (ø)

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[logandk]

Thanks a lot for the PR! I'm very sorry about the long response time, but I have not been able to free up time for this project in a while.